Or without modules:

($sec,$min,$hour,$mday,$mon,$year,$wday,$yday,$isdst) = localtime(time
+);  
$year += 1900;
$mon += 1;
print join '_', ($year,$mon,$mday,$hour,$min,$sec);  # YYYY_MM_DD_HH_M
+M_SS
[download]

Comment: It does not print values less than 10 with a leading zero. If you need that, it can be easily added. It is left as an exercise for the reader.

Update: Added $mon += 1. Thanks graff